Near-InfraRed Spectroscopy (NIRS) is an extremely useful technique of evaluating tissue metabolism, and NIRS is also applied in clinical practice. There is known a technique in which a living body such as a human body is irradiated with near-infrared light and a reflected light transmitted trough the living body is analyzed to measure a change in amount of blood in the living body. The measuring technique is based on a technique of utilizing a difference in light absorption characteristic between oxygenation and deoxygenation of the hemoglobin to detect a blood distribution state, thereby detecting a hemoglobin existing state.
Examples of NIRS include a continuous light method, a time resolved method, spatially resolved method, and an intensity modulation method. In each technique, when the deep layer tissue such as a muscle tissue and a brain is measured, a superficial tissue such as fat has a large influence on quantitative performance. The living body is usually formed by plural tissues, and the tissues have different absorption characteristics for the near-infrared light. Therefore, the analytical result of the reflected light includes information on the plural tissues.
The continuous light method and the spatially resolved method can be realized with a simple apparatus, and the continuous light method and the spatially resolved method have advantages in general versatility, portability, and real-time performance than any other methods. However, although a method for correcting an influence of a fat layer is proposed in NIRS in which the continuous light method is adopted (for example, see Non-Patent Documents 1 and 2), a method for correcting the influence of the superficial tissue is not sufficiently established yet in the spatially resolved method.
Although some investigations describe estimation of an absorption coefficient from a spatially resolved profile (for example, see Non-Patent Documents 3 to 5), there is shown no specific correction method which can easily be utilized for actual measurement of muscle tissue oxygen concentration. In addition to an error of absolute amount of hemoglobin concentration, it is also necessary to clarify an error in computing an oxygen saturation. Other investigation results are also reported (for example, see Non-Patent Documents 6 to 10).
Non-Patent Document 1: Yamamoto K, Niwayama M, Shiga T et al: Accurate NIRS measurement of muscle oxygenation by correcting the influence of a subcutaneous fat layer. Proc SPIE, 1998, 3194: 166-173.
Non-Patent Document 2: Niwayama M, Lin L, Shao J et al: Quantitative measurement of muscle hemoglobin oxygenation using near-infrared spectroscopy with correction for the influence of a subcutaneous fat layer. Rev Sci Instrum, 2000, 71: 4571-4575.
Non-Patent Document 3: Kienle A, Patterson M S, Dognitz N et al: Noninvasive determination of the optical properties of two-layered turbid media. Appl Opt, 1998, 37: 779-791.
Non-Patent Document 4: Fabbri F, Sassaroli A, Henry M E et al: Optical measurements of absorption changes in two-layered diffusive media. Phys Med Bio1, 2004, 49: 1183-1201.
Non-Patent Document 5: Shimada M, Hoshi Y, Yamada Y: Simple algorithm for the measurement of absorption coefficients of a two-layered medium by spatially resolved and time-resolved reflectance. 2005, Appl Opt, 44: 7554-63.
Non-Patent Document 6: van der Zee P, Delpy D T: Simulation of the point spread function for light in tissue by a Monte Carlo method. Adv Exp Med Biol, 1987, 215: 179-191.
Non-Patent Document 7: Wan S, Anderson R R, Parrish J A: Analytical modeling for the optical properties of skin with in vitro and in vivo applications. Photochem Photobiol, 1981, 34: 493-499.
Non-Patent Document 8: Mitic G, Kozer J, Otto J et al: Time-gated transillumination of biological tissues and tissue like phantoms. 1994, Appl Opt, 33: 6699-6710.
Non-Patent Document 9: Zaccanti G, Taddeucci A, Barilli M et al: Optical properties of biological tissues. 1995, Proc. SPIE, 2389: 513-521.
Non-Patent Document 10: Matcher S J, Elwell C E, Cooper C E et al: Performance Comparison of Several Published Tissue Near-Infrared Spectroscopy Algorithms. Anal Biochem, 1995, 227: 54-68.